It will be seen that all of them agree more or less closely as to the essential ingredients being uric acid and soda. According to Ebstein and Sprague they consist usually of almost pure _biurate_ of _sodium_ and _potassium_. But, as a rule, after a time _calcium_ salts are deposited. Dunin, it may be noted, has found deposits resembling gouty tophi, which contained only calcium salts. Kahn, again, claims that tophi do not always consist solely or even largely of urates, but that these may be replaced by _calcium_ salts. It may be added, too, that M. B. Schmidt has recorded, under the designation of “Calcium gout,” a case in which there existed a generalised deposition of calcium, and this in tissues other than those usually involved in “metastatic calcification.” But, to sum up, although there may be admixture of lime salts and organic matter, the salient chemical constituent of tophi is _biurate of sodium_. MODE OF FORMATION Gouty tophi, like all pathological concretions, are laid down in accordance with a definite law. In the first instance, a central nucleus is essential. To this must be added a “binding substance” or structural framework of different nature from the main mass of the concretion. Garrod, discussing the intimate structure of “chalk-stones,” observes that, “the large amount of phosphate of lime occasionally met with is probably derived not only from the tissue in which the chalk-stones have been developed, but likewise from secondary deposition, the result of ordinary inflammation around the original nucleus (urate of soda) which acts as a foreign body.” It is, however, quite possible that some substance other than _urate of soda_ constitutes the primary nucleus, for, as we now know, concretions most frequently gather around masses of mucin, clumped bacteria, desquamated cells, precipitated proteins, etc. Thus, the renal _uric acid infarcts_, supposed to result from disruption of the nucleo-proteins of the _fœtal nucleated red corpuscles_, take origin around injured _epithelial_ cells, which latter form the nucleus. As to _gouty tophi_, too, it has been suggested that they form in response to any _toxin_, resistance to which may involve death of the tissue cells with consequent disruption of their _nucleins_ and formation of _urates_. Such was the view held by Woods. Hutchinson, who also thought that the calcareous accretions might be regarded as “protective,” analogous to the formation of shells in the invertebrates, the process here consisting in the deposition of lime salts in cells already saturated with uric acid and urates.